You can use code such as following to get the tasks owned by an user.
BlockingTaskSummaryResponseHandler responseHandler = new BlockingTaskSummaryResponseHandler();
client.getTasksOwned(idRef, "en-UK", responseHandler);
tasks = responseHandler.getResults();
You can look at some sample code at http://community.jboss.org/people/bpmn2user/blog/2011/09/21/jbpm5-web-example